1. Field of the Invention
This application relates to apparatus and methods for optical fibers including, for example, optical fibers having a high numerical aperture.
2. Description of the Related Art
Significant technology developments in the late 1970s reduced optical fiber transmission loss to below 0.2 dB per km. These developments allow transmission of an optical signal over a few hundred kilometers without a repeater to boost signal, which is a significant achievement over electric cables based on metallic conductors. In addition to fiber transmission loss, another important transmission characteristic of an optical fiber is dispersion, which describes wavelength-dependent transmission delays. An optical pulse broadens during transmission in the presence of dispersion. This limits transmission speed as well as distance, because optical pulses will run into each other at the output end of the optical fiber at high transmission speed and long distance. The overlapped optical pulses at the output of the transmission fiber may not be recovered, and the transmitted information will be lost.
Dispersion in an optical fiber has two components: material dispersion and waveguide dispersion. Material dispersion comes from the wavelength-dependent refractive index of the material used to make optical fibers. For example, fibers may be made from a glass such as fused silica, with possible addition of dopants such as germanium, phosphorous, fluorine, and/or boron. Since the glass is mainly silica, material dispersion varies slightly from fiber to fiber. Waveguide dispersion on another hand comes from wavelength-dependent guiding properties of the optical waveguide and can be varied significantly by a waveguide design. Material dispersion and waveguide dispersion can have different arithmetical signs, and they can substantially cancel each other in some fiber embodiments and enhance each other in other embodiments. Standard silica, single-mode fiber has a zero-dispersion wavelength of ˜1.3 μm, which is the wavelength at which material and waveguide dispersion cancel each other precisely. An earlier generation of single-mode optical fiber transmission systems is based on operation at a wavelength of ˜1.3 μm, which provides low dispersion for high speed and long distance transmission.
Since the minimum transmission loss in silica fiber is at a wavelength of ˜1.55 μm, dispersion-shifted optical fibers with a zero-dispersion wavelength of ˜1.55 μm were developed in the mid-eighties by varying the waveguide dispersion. In the nineties, wavelength-division multiplexed (WDM) systems were developed to accommodate the rapidly growth of the Internet. In a WDM system, multiple channels located at different wavelengths are transmitted in the same fiber. Hundreds of channels can be used in a WDM system with components at the input and output respectively to multiplex and de-multiplex the large number of channels. In a WDM system, many channels may operate at wavelengths where there is significant dispersion. In addition, it was found that the large number of channels can increase optical intensity in the optical fiber to the point where different channels start to interact through nonlinear effects such as four-wave-mixing (FWM). It was also realized that a certain amount of dispersion can significantly reduce nonlinear effects such as FWM, because different channels can walk off each other in the presence of dispersion. This effectively reduces interaction length.
Thus, many optical fibers are fabricated to have a certain amount of dispersion and a large effective area to reduce nonlinear effects in WDM systems. Systems using such fibers typically use dispersion compensation modules (DCM) at repeaters to compensate for pulse broadening caused by dispersion. A DCM may comprise a few kilometers of dispersion-compensating fiber (DCF), which is an optical fiber designed to have high level of waveguide dispersion with an opposite sign to that of the transmission fiber. However, an additional issue in a WDM system is dispersion slope, which depends on the variation in dispersion with wavelength. A disadvantage of many dispersion-compensating fibers is that it is difficult to design a DCF with a high dispersion and appropriate dispersion slope to fully compensate dispersion for all the WDM channels. This sets a limit on the number of channels that can be used so that the channels at the two edges of the transmission windows do not suffer significant performance degradation due to residue dispersion.
Fiber chirped pulse amplification (FCPA) systems are often used for producing high peak power optical pulses. In an FCPA system, initial low-power optical pulses are stretched in time by a stretcher before amplification. The longer pulses have lower peak power so that the pulses may incur less nonlinear penalty in an optical fiber amplifier. After amplification, the longer pulses are then compressed to produce higher peak power optical pulses in a subsequent compressor. A stretcher may comprise an optical fiber with dispersion. More stretching is desired for higher peak power pulses. Stretching may be quantified by the stretching ratio, which is the ratio of the stretched pulse width of the pulse output from the stretcher compared to the pulse width of the pulse input into the stretcher. This stretching ratio is currently limited by the ability of current stretchers to match precisely the dispersion and dispersion slope of the compressor. A stretcher which can better match the terms of higher order dispersion with that of the compressor can further improve current FCPA systems and produce more compressed pulses.
In addition to dispersion, loss is another important parameter in fibers, specifically in systems having a total loss budget. Designs with reduced or minimum loss penalty are strongly favored.
Thus, what is needed is optical fiber that can provide a desired dispersion and that can also provide a suitable dispersion slope at a small optical loss. The present disclosure describes apparatus and methods providing optical fibers that can be used in applications such as described above as well as in other applications.